Human Diploid Cell Rabies Vaccine was licensed for intramuscular use in the United States in 1980. Six years later the vaccine was licensed for intradermal use for pre-exposure immunization and booster doses only. All of the studies conducted prior to licensure indicated that the vaccine was highly potent and virtually every person vaccinated seroconverted. Because of the confidence in the vaccine, the Advisory Committee for Immunization Practices in their Recommendations for Use of Rabies Vaccine suggested that testing for antibodies was only necessary for those persons at continuous risk of infection and this was to be done every 6 months, and for persons at frequent risk sera was to be tested every two years or a booster dose given. CDC in reviewing data collected for another study found several persons immunized with the pre-exposure regimen by the intradermal route did not have a titer within one year of immunization. We are working with CDC and the manufacturer to analyze this finding. We will be testing the potency of the vaccines associated with the non- response and the counterpart vaccine made from the same bulk but produced for the intramuscular route. In addition, a questionnaire has been formulated to use in a telephone interview of cases. A case is defined as anyone receiving a rabies pre-exposure immunization course by the intradermal route whose titer is <1:5 within one year of completion of the immunization. If potency of the vaccine is not the issue we will explore host factors and technique of administration of the intradermal injection. It is not clear at this time if the individuals without detectable antibody were primed or whether the antibody had merely waned; or if these individuals failed to respond to the rabies vaccine. After an investigation into these adverse reactions (i.e. non-response) a decision will be made as to whether serological follow-up should be part of intradermal immunization.Consideration will be given to determine if intradermal inoculation should be restricted to booster doses only.